It is widely known in the art that metal-oxide semiconductor (MOS) capacitors provide superior performance over other capacitor structures in integrated circuitry. One area where MOS capacitors are superior is in charge integrity when the capacitor is struck by an alpha particle. This is particularly important in dynamic random access memories (dRAMs). Alteration of the charge on the storage capacitor in a dRAM cell causes an error in the data stored. This is one form of soft error and is a widely recognized problem. MOS capacitors are less susceptible to soft errors because the alpha particle must impart enough energy to an electron to bring the electron up to conduction band of the oxide, which is very high.
One recently developed embodiment of an MOS capacitor is the trench capacitor. This capacitor is formed by etching a cavity (the trench) in the surface of a substrate, forming an insulator on the sides of the trench and filling the trench with a conductive material. One plate of the capacitor is formed by the conductive material in the trench, and the other by the substrate. In order to provide the above mentioned soft error characteristic, the charge must be stored on the inner conductive layer which is electrically isolated from the substrate by the insulator. Electrical contact to the conductive layer can be made at the top of the the trench by the usual integrated circuit interconnect techniques. However, a better method for contacting the conductive layer allows the use of the upper portion of the trench for other purposes. One such purpose is the incorporation of a pass transistor formed in the upper portion of the cavity which is connected to the capacitor, thus forming a dRAM cell in a trench. A type of trench dRAM cell is the subject of the parent application of this case.
A problem with the trench cell arrangement is that the capacitor and the transistor cannot be accessed separately. The capability for accessing the separate components is important in characterization of the components during development and manufacture of an integrated circuit using the trench cell and is useful in circuit design.